1. Field of the Invention
The present invention relates to a synthetic polyvinyl alcohol (hereinafter sometimes referred to as PVA) fiber that has excellent mechanical features including high strength, high elastic modulus and abrasion resistance, can readily be pulpified. In particular, the invention relates to a synthetic PVA fiber that can be used in the industrial fields including reinforcement for composite materials, as well as in the fields of synthetic paper and replacement for asbestos.
2. Description of the Prior Art
PVA fiber has higher strength and elastic modulus than other general-purpose fibers, and has widely been used under a commercial name of "Vinylon" principally in the industrial field. In recent years it has also been used for reinforcing cement, as a replacement for asbestos. However, with the recent trend for requiring industrial materials exhibit still higher performance, there has also been increasing demand for PVA fiber with still higher strength and elastic modulus and with the capability of being pulpified, i.e. formed into extrafine fibrils, like asbestos.
Experience with polyethylene proved that synthetic fibers with high strength and elastic modulus can be obtained by, besides employment of rigid liquid crystal polymers, conducting gel spinning of flexible general-purpose polymers with super-high molecular weights. Attempts have since been made to obtain high-performance fibers from general-purpose polymers. Thus, Japanese Patent Application Laid-open Nos. 100710/1984, 130314/1984, 108711/1986, etc. disclose techniques for producing PVA fiber with strength and elastic modulus considerably higher than conventional PVA fiber. However, the performance level of the fiber obtained by this technique does not yet reach that of superdrawn polyethylene fiber. The difference is considered to be due to the presence of strong intermolecular hydrogen bond in PVA. Where conventional gel spinning is employed, PVA fiber becomes whitened by drawing up to a ratio of 20 or so, and, if drawn more, the fiber will start decreasing in strength.
Conventional PVA fiber has been used as, making use of its high strength and hydrophilic property, replacement fiber for asbestos in the field of cement reinforcement and the like. It however has a problem in formability because it has a diameter as large as more than 10 times that of asbestos. That is, in the process of forming slate and the like, if a reinforcing fiber has a large diameter, it will not sufficiently catch cement particles and hence will need to be mixed with natural pulp or the like. In the formation of brake disks or the like, PVA fiber which is not pulpified catches the resin to be reinforced only insufficiently as compared with asbestos, thereby decreasing the strength of green material. It has therefore been difficult to replace asbestos in this field by conventional PVA fiber. In the field of synthetic paper also, pulpified PVA fiber having thinner fineness would produce higher grade paper.
Spinning of high-performance synthetic fiber through a spinneret having microfine holes has been attempted only to prove there is a limit of fineness attainable by physical finization. There has also been desired a fiber that pulpifies first when thrown into a wet refiner, since pulpified fiber having the shape of separate short-cut filaments is difficult to handle during processes prior to the wet refinery.
In consideration of the foregoing, an object of the present invention is to provide a synthetic PVA fiber that can be superdrawn and has excellent mechanical properties, and can be pulpified.
Another object of the present invention is to provide a synthetic PVA fiber having the above characteristics and suffering from no whitening.
The present inventors thought the fact that a single filament consists of infinite number of fibrils can make it possible to realize high strength and elastic modulus by superdrawing, and also thought that the very fact could make it possible to pulpify the filament. To realize the idea in PVA fiber, the present inventors have discovered improvements in the dope stage of the fiber and found a process that can make the fiber be formed of an aggregate of fibrils already at the stage of as-spun (before heat drawing) fiber, to complete the invention.